Hello Danny,
As others have stated, the plate numbers really depend on when a car was first sold or transferred into the state and not so much on when it was built.
Here's my two cents worth of black plate info and maybe a suggestion or two....
Even though the Y series plates that you have were once assigned to your car, in my recent experience it's been difficult to get DMV to work with plates that are not "clear". It would be cool if you can them reassigned to your car, but it may be more of a challenge than you wish to deal with.
I've probably had at least 30 original local Bay area 1968 cars with W series black plates, but only a few with the X series. There is one local 68 RS Z that was special ordered by a guy in San Rafael (through a Point Reyes dealer - Cheda Chevrolet) that was built a little later than yours and has plates with "WNA" series. I would use that as a high letter target when choosing a set of plates if you go that direction.
It might be easier for you to get a set of clear W series plates and have then assigned to your car. I may even have an extra set you can use... If you do. you might also get less grief from the local "experts" who see your car with the Y series plates and tell you they are wrong (like I get on my 67 Corvette which has Y series plates).
I could probably give you some more accurate letter series suggestions based on some of my old cars, but my records are at home and I'm currently in Korea. I'll chat with you when I get back.
BTW - any progress on your distributor? I may need mine back some time in the next couple of months so I can get that Z fired up and dialed in.
Talk to you soon, Charlie